Blending machinery for mixing a plurality of different kinds or colors of plastic materials is well known. In such machinery selected quantities of plastic materials are fed from two or more supply hoppers to a receiving or weigh hopper through valve controlled delivery means. The quantities of plastic materials discharged to the weigh hopper are monitored by various mechanisms which function to indicate the approximate amount of such materials present in the weigh hopper.
A major disadvantage of conventional blending machinery of the type just described is the inability thereof to meter accurately the exact quantity of material discharged from each supply hopper. For example, even in those instances in which the sensing means of conventional machinery is operable to detect the delivery of a selected quantity of plastic material to the weigh hopper, there usually is such a time delay between the detection and the termination of delivery that an excess of material is deposited into the weigh hopper. Thus, the quantity of material contained in the weigh hopper rarely, if ever, corresponds to the optimum quantity.